Bonded foil strain gauge (BFSG) pressure transducers & transmitters have metal foil gauges made from materials such as Constantan which are bonded using industrial adhesives to a stainless diaphragm or beam which is the material which flexes in response to change in pressure.
The foil strain gauge sensing element typically includes two or four foil strain gauges which are joined together into a Wheatstone bridge circuit.
These type of pressure sensors are most suited to mid to high range pressure measurement and because the sensing diaphragm is stainless steel they are very robust and can withstand high dynamic pressure changes.
Products
TPF Flush Diaphragm Strain Gauge Pressure Sensor - Flush diaphragm piezo-resistive pressure sensor for food processing and plastic injection moulding applications with a millivolt output signal in pressure ranges from 10 up to 1000 barg.
DMP304 Ultra High Range Hydraulic Pressure Transducer - Current output pressure transducer for measurement ranges from 0 to 2000 (30000 psi) range up to a maximum range of 6000 bar (90000 psi).
TPHADA Ultra High Range Pressure Sensor
TPS Strain Gauge Pressure Sensor
TPSA Precision High Pressure Transducer
DMP 335 All Welded Stainless Steel Diaphragm Pressure Sensor
Applications
30000 psi water and cutting oil pressure transmitter - High pressure transmitter for use in hydraulic circuit to measure a composition of water mixed with cutting oil. This sensor is zero & span adjustable via external potentiometers.
60000 psi hydraulic pressure transducer with 0-5Vdc signal out - 0-5 volt output pressure transducer with all welded stainless steel construction for measuring very high hydraulic oil pressures up to 60,000 psi.
40 ksi g 4-20mA freshwater pressure sensor plus display for hydrothermal testing use - A combined ultra high range gauge pressure sensor and digital display for hydrothermal testing use to measure pressure of freshwater over a range of 0 to 40 ksi g from the 9/16 UNF female (AE HP type) process connection, and sending the corresponding 4-20mA signal through the DIN plug electrical connection.
50000 psig 4-20mA mineral hydraulic oil pressure sensor for chemical processing use - An intrinsically safe ultra high range pressure sensor for chemical processing use to measure pressure of mineral hydraulic oil over a range of 0 to 50,000 psi g from the 9/16 UNF female process connection, and sending the corresponding 4-20mA signal through the integral cable electrical connection.
Oil field equipment testing 20,000 psi g 4-20mA out freshwater pressure sensor
60000 psig range 0-10Vdc output engine oil pressure sensor for high pressure homogenization use
Cold isostatic press 5000 barg 0-10Vdc output high range freshwater pressure transducer
5000 bar Pressure Transmitter with AE HP fitting
Compression machine 3mV/V output 700bar range pressure transducer
10000 psig range 4-20mA output mineral hydraulic oil pressure sensor for hydraulic press use
15000 psig range 0-5Vdc output freshwater pressure sensor for waterjet cutting research use
How It’s Made
Bonded foil strain gauges are manufactured using photo-etching, similar to printed circuit boards. This process involves designing the gauge pattern, transferring it to a metal foil using a photosensitive film, then etching away the unwanted areas. The resulting strain gauge is carefully placed on the diaphragm with adhesive and cured under controlled conditions. This bonding process is crucial for ensuring accurate and reliable pressure measurements, as a strong and stable bond protects the gauge from external influences.
How it works
BFSG pressure sensors operate on the principle of measuring the deformation of a diaphragm or beam caused by pressure changes. A thin foil strain gauge, typically made of a metal alloy like Constantan, is bonded to the diaphragm’s surface. When pressure is applied, the diaphragm flexes, causing the strain gauge to stretch or compress. This deformation alters the electrical resistance of the strain gauge, which is then measured using a Wheatstone bridge circuit. The change in resistance is directly proportional to the applied pressure, allowing the sensor to convert the mechanical deformation into an electrical signal that can be easily processed and interpreted.
Characteristics & Properties
Bonded foil strain gauge technology, while originating in the mid-20th century, predates many modern pressure sensing technologies like piezoresistive and capacitive sensors. However, it continues to hold a significant position in the industry due to a combination of factors.
Cost-Effectiveness
The manufacturing process for bonded foil strain gauges, primarily photo-etching, is well-established and relatively inexpensive compared to newer technologies. This results in lower production costs, particularly for high-volume applications, making them an attractive option for price-sensitive markets.
The materials used in bonded foil strain gauges, such as Constantan for the foil and stainless steel for the diaphragm, are relatively inexpensive and readily available. This further contributes to the overall cost-effectiveness of the sensors.
Proven Reliability
Decades of use in various industries have demonstrated the reliability and longevity of bonded foil strain gauges. Their simple and robust design, with minimal moving parts, translates to fewer failure points and a longer operational lifespan. This proven track record makes them a trusted choice for applications where reliability is paramount.
Simple and Robust Design
The design of bonded foil strain gauges is inherently simple, consisting of a thin metal foil etched with a strain-sensitive pattern and bonded to a diaphragm. This simplicity translates to fewer components, less complex assembly processes, and reduced labour costs.
The straightforward design of bonded foil strain gauges contributes to their durability and ease of use. With no complex electronics or delicate components, they can withstand harsh environments and challenging operating conditions.
High Shock & Vibration Tolerance
The adhesive used to bond the strain gauge to the diaphragm is specifically designed to withstand high stresses and strains, including those induced by shock and vibration. This strong bond ensures that the gauge remains securely attached to the diaphragm and continues to function accurately even under extreme conditions.
The materials used in the sensor, such as stainless steel for the diaphragm and Constantan for the strain gauge, possess excellent mechanical properties, including high tensile strength and fatigue resistance. This enables them to withstand repeated cycles of stress and strain without failure.
Limitations
Accuracy & Repeatability
While BFSG pressure sensors offer numerous advantages, including robustness and cost-effectiveness, their accuracy and repeatability can be lower compared to some competing technologies, primarily due to limitations in materials and bonding processes:
Hysteresis and Creep
The bonding adhesive used in these sensors can exhibit hysteresis and creep over time. Hysteresis refers to the difference in output readings when the pressure is increasing versus decreasing, while creep refers to the gradual change in output under constant pressure. Both phenomena can introduce errors in measurement and affect the sensor’s repeatability.
Temperature Effects
Although bonded foil strain gauges can be temperature compensated, the bonding adhesive and the diaphragm material can still experience thermal expansion and contraction. These temperature-induced changes can affect the sensor’s sensitivity and zero offset, leading to variations in accuracy and repeatability across different temperature ranges.
Bonding Imperfections
The bonding process itself can introduce inconsistencies. Variations in adhesive thickness, curing conditions, and surface preparation can lead to non-uniform stress distribution in the strain gauge, resulting in measurement errors and reduced repeatability.
Material Limitations
The materials used in bonded foil strain gauges, while durable and cost-effective, may not offer the same level of sensitivity and stability as more advanced materials like silicon or thin-film strain gauges. These newer materials can exhibit lower hysteresis, better temperature stability, and higher gauge factors, resulting in improved accuracy and repeatability.
Ageing Effects
Over time, the bonding adhesive can degrade due to exposure to environmental factors like temperature, humidity, and chemical contaminants. This degradation can weaken the bond between the gauge and the diaphragm, leading to shifts in the sensor’s output and decreased repeatability.
Manufacturing Time
BFSG pressure sensors can experience longer delivery lead times compared to newer technologies due to several factors related to their manufacturing process:
Photo-etching Process
The primary method for manufacturing bonded foil strain gauges involves photo-etching, which is a multi-step process that can be time-consuming. Each step, from creating the photomask to etching the foil, requires careful handling and precise timing to ensure accuracy and quality. This can lead to extended production cycles, particularly for complex or custom designs.
Manual Manufacturing Processes
While some aspects of the manufacturing process are automated, many steps still require manual operations, such as positioning and aligning the foil during bonding. This reliance on human intervention can introduce variability and potential delays, especially in high-volume production scenarios.
Yields
The photo-etching process can result in lower yields compared to newer manufacturing techniques like micromachining or thin-film deposition. Imperfections in the photomask, etching inconsistencies, or handling errors can lead to defective strain gauges, requiring rework or discarding, which further extends lead times.
Output Sensitivity
Metal foil strain gauge pressure sensors generally exhibit lower output sensitivity compared to other technologies, such as piezoresistive or capacitive sensors, due to the characteristics of the strain gauges themselves.
Gauge Factor
The gauge factor, which is a measure of the strain gauge’s sensitivity to strain, is typically lower for metal foil strain gauges compared to semiconductor-based strain gauges used in piezoresistive sensors. This means that for a given amount of strain or pressure change, the change in resistance of the bonded foil strain gauge is smaller, resulting in a weaker electrical signal output.
Signal-to-Noise Ratio
The lower output signal from bonded foil strain gauges can make them more susceptible to noise interference, especially in applications with low-pressure ranges or high electrical noise environments. This can affect the overall accuracy and precision of the measurement.
Overpressure Rating
Bonded strain gauge pressure sensors can have a lower overpressure rating compared to some other pressure sensing technologies due to the inherent limitations of the metal diaphragms they utilize:
Elastic Limit
Metal diaphragms, while robust, have a finite elastic limit. This means they can only flex or deform to a certain extent before undergoing plastic deformation, which is permanent and can damage the sensor. If the applied pressure exceeds this elastic limit, the diaphragm may not return to its original shape, leading to inaccurate readings or complete sensor failure.
Fatigue
Repeated exposure to overpressure conditions can lead to fatigue in the diaphragm material. This is a weakening of the material due to cyclical stress, which can eventually cause cracks or fractures, rendering the sensor inoperable.
Creep
Even within their elastic limit, metal diaphragms can exhibit creep, which is a gradual deformation under constant pressure. This can lead to a slow drift in the sensor’s output over time, affecting its accuracy and long-term stability.
Thickness Limitations
To achieve higher overpressure ratings, thicker diaphragms are required. However, thicker diaphragms are less sensitive to pressure changes, resulting in lower output signals and reduced accuracy. Therefore, a trade-off exists between overpressure rating and sensitivity in bonded resistance strain gauge pressure sensors.
Calibration Drift
Bonded metal strain gauge pressure sensors can experience calibration shifts over their service life due to the cumulative effects of fatigue on both the diaphragm material and the bonding adhesives. This phenomenon is exacerbated by operational pressure and temperature cycling, leading to a gradual degradation of the sensor’s accuracy and reliability.
Diaphragm Fatigue
The diaphragm, typically made of stainless steel, undergoes cyclic stress as it flexes under pressure changes. Over time, this repeated stress can cause microscopic cracks or changes in the material’s crystal structure, weakening the diaphragm and altering its response to pressure. This can manifest as a shift in the sensor’s zero offset or sensitivity, leading to inaccurate readings.
Adhesive Degradation
The bonding adhesive, which secures the strain gauge to the diaphragm, is also susceptible to fatigue. Temperature fluctuations and pressure cycles can cause the adhesive to expand and contract, leading to stresses that weaken the bond over time. This can result in slippage of the strain gauge, changes in the strain transfer efficiency, and ultimately, calibration shifts.
Thermal Cycling
Repeated exposure to temperature variations can accelerate both diaphragm fatigue and adhesive degradation. The different thermal expansion coefficients of the diaphragm material, adhesive, and strain gauge can create additional stresses during heating and cooling cycles, further contributing to the overall degradation of the sensor’s performance.
Pressure Cycling
Similar to thermal cycling, pressure cycling subjects the diaphragm and adhesive to repeated stresses. Each pressure cycle causes the diaphragm to flex, and the adhesive to experience shear forces. Over time, these stresses can accumulate and lead to material fatigue, resulting in calibration shifts.
Advantages
- Robust for high pressure range applications such as hydraulics systems
- High reliability from straightforward sensor design refined over many decades
- Economical pricing due to lower cost of manufacture, less complex assembly processes.
- High shock and vibration tolerance due to material properties of metal diaphragm and the high strength of strain gauge bonding to the diaphragm.
Disadvantages
- Accuracy and repeatability performance lower than competing sensor technologies due to the less advance materials and bonding processes employed to manufacture
- Longer delivery lead times due to older manufacturing processes, lower yields
- Lower output sensitivity due to lower impedance strain gauges than other technologies.
- Lower overpressure rating due to limitation of elasticity of metal diaphragms.
- Calibration shifts over time due to fatigue of diaphragm materials and strain gauge bonding adhesives from operational pressure & temperature cycling over the service life.
Checklist for bfsg pressure sensor requirements
Define your bonded metal foil strain gauge pressure sensor requirements using this checklist:
- Pressure range?
- Overpressure?
- Output signal?
- Dynamic response?
- Accuracy?
- Electrical connection?
- Process connection?
- Size dimensions?
- Process media type?
- Media temperature range?
- Environment temperature range?
- Environment shock and vibration?
- Shock pressures?